The use of digital data communication has become widespread to the point of nearly being ubiquitous. For example, digital communications are routinely implemented in providing data communications in various systems, such as computer network systems (including personal area networks (PANs), local area networks (LANs), metropolitan area networks (MANs), wide area networks (WANs), the Internet, etc.) and communication network systems (including the public switched telephone network (PSTN), cellular networks, cable transmission systems, etc.).
A network may operate in synchronous mode (e.g., time-division duplexing (TDD) mode), in which transmissions by different network devices (e.g., base stations, access points, user equipment, mobile devices, etc., performing wireless communications utilizing various forms of receivers, transmitters, and/or transceivers) within the network are aligned in time and/or frequency. In such a synchronous network, synchronization of the network devices is of utmost importance because if a network device is not accurately synchronized to the rest of the network (e.g., synchronized to other network devices), its transmissions will not be aligned with the transmission of the other network devices. This misalignment may result in network degradation, as the misaligned network device may cause, when transmitting, interference to other network devices. Also, the misaligned network device may be subjected to interference when other network devices are transmitting. In 4th Generation (4G)/long term evolution (LTE) wireless communication system, for example, the 3GPP TS36.133 standard requires that synchronization of network devices of a TDD network must be within 3 us.
Synchronizing the network devices may be accomplished by providing the network devices with synchronization signals, which the network devices can use for performing the synchronization. The synchronization signals may be provided to the network devices from a common synchronization source (hereinafter referred to as “global synchronization source”). The global positioning system (GPS) provides timing reference signals that may be utilized as a global synchronization source by network devices having a GPS receiver.
In some network deployments, a particular network device may not be able to receive, or reliably receive, synchronization signals from the global synchronization source. For example, GPS reference signals, as may be utilized as global synchronization source synchronization signals, are often only reliably received when a device is provided with a relatively unobstructed view of the sky (e.g., disposed outdoors, in an area free of shadowing from terrain, foliage, and structures, etc.). Accordingly, network devices disposed indoors, in an area shadowed by tall buildings or terrain, etc., may be unable to reliably receive GPS reference signals for establishing synchronization with other network devices. A network device that is unable to reliably receive synchronization signals from a global synchronization source may, nevertheless, be within communications range of other network devices, wherein one or more such network devices may be capable of receiving synchronization signals from the global synchronization source. The network devices that are able to receive, or reliably receive, synchronization signals from the global synchronization source may thus broadcast synchronization signals for use by the network devices that are unable to reliably receive the synchronization signals from the global synchronization source. Accordingly, the network devices that are unable to reliably receive synchronization signals from the global synchronization source are able to nevertheless synchronize their communications to the other network devices by using a synchronization signal broadcast by another network device (the particular network device broadcasting the synchronization signal selected/used by another network device being referred to herein as a “synchronization target” and a “parent network device” for the network device receiving the broadcast synchronization signal and the network device using the broadcast synchronization signal being referred to herein as a “child network device”).
The same approach can be used for any number of network devices that cannot reliably receive synchronization signals from the global synchronization source but are within communications range of other network devices that are able to reliably receive synchronization signals from the global synchronization source. Moreover, network devices reliably receiving broadcast synchronization signals from a parent network device, and thus establishing network synchronization, may themselves broadcast, or rebroadcast, synchronization signals such that other network devices that are within communications range of these network devices are able to receive synchronization signals. In this case, the network devices receiving these broadcast synchronization signals can use those network devices broadcasting synchronization signals as synchronization targets and the network devices broadcasting the synchronization signals themselves are parent network devices to the network devices receiving the broadcast synchronization. Although various network devices are not receiving the synchronization signals directly from the global synchronization source, all of the network devices nonetheless can be synchronized to the other devices in the network via their respective synchronization target, either by synchronizing to the global synchronization source using synchronization signals broadcast by a network device receiving the synchronization signals directly from the global synchronization source or using synchronization signals relayed by one or more network devices.
It should be appreciated from the foregoing that, in some cases, the synchronization targets themselves may not be receiving synchronization signals directly from the global synchronization source, but may be instead receiving synchronization signals broadcast by an intermediate, intervening network device serving as their synchronization target. Thus, this interconnection of the various network devices to each other as parent and child network devices to provide a synchronization target hierarchy for providing synchronization signals to the variously disposed network devices may create a synchronization tree, with the various network devices as network devices of the tree. Network devices in the synchronization tree may be children network devices of their respective synchronization target parent network devices, and synchronization target parent network devices may be parent network devices to their respective children network devices. Furthermore, a network device may have a child network device that itself has children network devices. The child network device and the children network devices thereof are said to be downstream of the parent network device, and are said to be downstream children of the parent network device.
As can be appreciated from the aforementioned synchronization tree structure, some network devices may be directly receiving synchronization signals from the global synchronization source and thus, may be said to be one hop away from the global synchronization source. These network devices are thus said to have a synchronization tree stratum level of one. Network devices receiving synchronization signals broadcast from a synchronization target in the synchronization tree are said to be more than one hop away from the global synchronization source due to the synchronization signal being provided by one or more intermediary, intervening network devices, wherein the number of hops depends upon the network device's placement in the synchronization tree hierarchy. Accordingly, a stratum level of such a network device indicates the number of hops that exist between the network device and the global synchronization source of the network.
A stratum index of a network device may be used to indicate the particular stratum level of the network device. Thus, network devices that are connected directly to the global synchronization source may be said to have a stratum level of 1, and may be configured with a stratum index of 1. A network device using a stratum level 1 network device as synchronization target may be said to be two hops away from the global synchronization source, i.e., one hop from the device to the synchronization target, and another hop from the synchronization target to the global synchronization source. Such a network device may thus be configured with a stratum index of 2. A network device using a stratum level 2 network device as synchronization target may be said to be three hops away from the global synchronization source, and may be configured with a stratum index of 3, and so on. Generally, the stratum index of a network device may be equal to the stratum index of its synchronization target plus a non-zero, positive number. The non-zero, positive number may indicate a distance between the network device and its synchronization target. For example, the stratum index of a network device may be equal to the stratum index of its synchronization target plus the distance, in hops, between the network device and its synchronization target.
In operation of a wireless network, any particular network device may fail. When this happens, any network device configured with the failed network device as its synchronization target, as well as any network devices downstream in the synchronization tree, will be in danger of losing synchronization. Thus, in this case, fast and efficient selection of a new synchronization target for the network device is of paramount importance. A number of techniques for selecting a new synchronization target have been utilized. However, these techniques have generally not been well suited for use with respect to some network scenarios and/or configurations. For example, the existing synchronization techniques are overly restrictive, and often ineffective, when operating under certain standard operational constraints.
One technique for selection of a synchronization target, as shown in U.S. Pat. No. 8,213,405, uses quality metrics for selecting or reselecting a synchronization target. A network device may be selected as a new synchronization target only if it has higher quality metrics than the current synchronization target. Thus, when a first device loses its synchronization target, it may only select a second device as a synchronization target if the quality metrics of the second device are higher than the quality metrics of the failed synchronization target. Such technique is not well suited for use in network situations where a network device with higher quality metrics than the failed synchronization target is not available, even if a network device with lower quality metrics that the failed synchronization target is available. In this case, this technique will not select the network device with lower quality metrics as synchronization target, even though it is available, and synchronization of the first network device will be lost. As such, this technique provides an overly restrictive solution that is not effective in all operational conditions.
Some techniques for selection of a synchronization target for a network device may not restrict selection of a new synchronization target to only those network devices having a lower stratum index than the stratum index of the network device. A first network device may synchronize to a second network device regardless of the second network device's stratum index. Such technique, however, is not suitable for network deployments that employ a global synchronization source. For example, the second network device may be a child network device of the first network device and thus, may have a higher stratum index than the first network device. The first network device may decide to synchronize to the second network device, not knowing that the second network device is its child network device, which is itself synchronized to the first network device. Thus, under these techniques, a first network device would not be precluded from synchronizing to its own child network device that is itself synchronized to the first network device. Hence, neither the first network device nor the second network device would be synchronized to the global synchronization source. As such, these technique provides a solution that is not effective in all operational conditions.